Development of a new compact gamma-ray spectrometer optimised for runaway electron measurements

Molin, A. Dal; Martinelli, L.; Nocente, M.; Rigamonti, D.; Abba, A.; Giacomelli, L.; Gorini, G.; Lvovskiy, A.; Muraro, A.; Tardocchi, M.; Contributors, Jet

doi:10.1063/1.5038803

Cited by 15 publications

(10 citation statements)

References 19 publications

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“…HXR measurements during the RE plateau regime are challenging even at a small RE current and a small high-Z impurity content due to still very high bremsstrahlung flux and saturation of conventional GRI gamma detectors based on BGO crystals. To overcome this limitation, a novel ultrafast gamma detector consisting of a LYSO crystal (Lu 1.8 Y 0.2 SiO 5 :Ce) coupled to a Multi-Pixel Photon Counter has been developed and implemented to the GRI central line of sight [29,[39][40][41][42]. This allowed increasing the time resolution by a factor of 1000x compared to the BGO detectors [37] and achieving MHz counting capabilities (discussion of the pile-up effect at the high counting rate can be found in appendix B).…”

Section: Details Of Measurementsmentioning

confidence: 99%

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

et al. 2020

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Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard X-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5-6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal MHD instability is accessed by ramping up the current. The instability leads to a sawtoothlike relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.

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Section: Details Of Measurementsmentioning

confidence: 99%

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

et al. 2020

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“…The RE energy distribution function is diagnosed via collimated tangential measurements of the HXR bremsstrahlung radiation from the central plasma using the gamma ray imager (GRI) [26][27][28] and inversion from HXR to RE spectra [29,30]. RE loss to the tokamak wall is measured using a distant HXR scintillating plastic detector [31].…”

Section: Experiments and Diagnosticsmentioning

confidence: 99%

Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

et al. 2019

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We report the first observation of chirping instabilities driven by runaway electrons (REs) in a tokamak. The instabilities are accessed during the post-disruption RE beam stage in a low density background plasma ( m−3) on DIII-D. The chirping instabilities are observed when a decelerating loop voltage is applied to the RE beam. The frequency chirping is detected in two distinct frequency bands: 0.1–10 MHz and 30–80 MHz. The mode frequency increases linearly when the toroidal magnetic field sensed by the RE beam increases. The frequency chirps by 0.3–2.4 MHz on a timescale of 1 ms. Modification of the RE distribution function is directly measured during the chirping in the low-frequency band consistent with the hole–clump model for frequency chirping. The low-frequency instabilities also correlate with an increase of intermittent RE loss from the plasma. These observations provide a novel experimental platform for fundamental studies of nonlinear chirping. They also support continued investigation of opportunities to utilize kinetic instabilities for RE mitigation in a tokamak reactor.

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“…This flux limit was compatible with quiescent scenario RE beams, but not the more intense post-disruption beams. The more recent work summarized in section 2.2 utilized a new scintillation crystal (LYSO), with ns-scale scintillation pulses enabling 10 7 counts s −1 per detector [46,47]. Future GRI development work is focused on increasing the number of these more advanced detectors deployed on the GRI to better diagnose post-disruption conditions.…”

Section: Re Distribution Measurement Via Bremsstrahlungmentioning

confidence: 99%

Recent DIII-D advances in runaway electron measurement and model validation

Paz-Soldan¹,

Eidietis²,

Hollmann³

et al. 2019

Nucl. Fusion

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Novel measurements and modeling of runaway electron (RE) dynamics in DIII-D have resolved experimental discrepancies and validated predictions for ITER, improving confidence that RE avoidance and mitigation can be predictably achieved. Considering RE formation, first experimental assessments of the RE seed current demonstrates that present hot-tail theories are not yet accurate and require improved treatment of the pellet dynamics. Novel measurements of kinetic instabilities in the MHz-range have been made in the RE formation phase, with the intensity of these modes correlated with previously unexplained empirical thresholds for RE generation. Controlled RE dissipation experiments in quiescent regimes have validated RE distribution function dependencies on collisional and synchrotron damping, both in terms of distribution function shape and dissipation rates. Measurements of RE bremsstrahlung and synchrotron emission are now used in tandem to resolve energy and pitch-angle effects. A resolution to long-standing dissipation anomalies in the quiescent regime is offered by taking into account kinetic instability effects on RE phase-space dynamics. Kinetic instabilities in the 100-200 MHz range are directly observed, though modeling finds the largest dissipation arises from GHz range instabilities that are beyond the reach of existing diagnostics. Kinetic instabilities are also observed in the mature post-disruption RE plateau phase, so long as the collisional damping rate is reduced with low-Z injection. Experiments with high-Z injection find that the dissipation rate saturates with injection quantity, likely due to neutral diffusion rates being slower than vertical instability rates in DIII-D. Considering the final loss, a 0D model for first-wall Joule heating is found to be in agreement with experiment, and controlled access to RE equilibria with edge safety factor of two identifies novel dynamics brought about by large-scale kink instabilities. These dynamics are typified by fast (tens of microseconds) RE loss rates without RE beam regeneration. The above measurements and comparison with theory represent significant advances in the understanding of RE dynamics and indicate possible new opportunities for RE avoidance or mitigation via kinetic instabilities.

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Development of a new compact gamma-ray spectrometer optimised for runaway electron measurements

Cited by 15 publications

References 19 publications

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

Recent DIII-D advances in runaway electron measurement and model validation

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